The present invention is generally directed to a method and an apparatus for measuring a lifetime of charge carriers in a semiconductor structure.
Integrated circuits comprise a large number of individual circuit elements such as e.g., transistors, capacitors and resistors. These circuit elements are connected by means of electrically conductive features to form complex circuits such as memory devices, logic devices, and microprocessors. In modern integrated circuits, the circuit elements may be formed on and in a so-called semiconductor on insulator structure. A semiconductor on insulator structure comprises a layer of a semiconductor material, for example a layer of silicon, which is formed over a layer of an insulating material provided over a substrate. The insulating material may, for example, comprise silicon dioxide and the substrate may comprise a silicon wafer. Properties of the circuit elements may be sensitive to characteristics of the layer of semiconductor material. In particular, the properties of the circuit elements may be influenced by the recombination lifetime of charge carriers (electrons and holes) in the layer of semiconductor material. Therefore, it may be desirable to measure the recombination lifetime of electrons and/or holes in the layer of semiconductor material.
In a method of measuring a recombination lifetime of charge carriers according to the state of the art, a semiconductor wafer is irradiated with microwave radiation. The reflectance of the wafer for the microwave radiation which is related to the concentration of charge carriers in the wafer, is monitored by measuring an intensity of the reflected microwave radiation. A laser pulse is applied to the wafer to generate electron-hole pairs in the wafer. Due to the generation of the electron-hole pairs, the concentration of charge carriers, in particular the concentration of minority carriers, is increased. Hence, excess minority carriers are created in the wafer. The generation of the excess minority carriers leads to an increase of the reflectivity of the wafer for the microwave radiation.
Thereafter, the excess minority carriers can recombine with majority carriers, which leads to a decay of the reflectivity of the wafer for the microwave radiation. A time constant of the decay, which may be related to the lifetime of the excess minority carriers, may be determined.
In one example of a method of determining a lifetime of charge carriers according to the state of the art, an exponential functionR(t)=A+Bexp(−t/τ1)may be fitted to measurement data obtained during a period of time during which the reflectivity decays approximately exponentially, wherein t denotes the time which has passed since the laser pulse and A, B and τ1 are parameters which are adapted such that R(t) represents the measured reflectivity values obtained during the period of time as close as possible. The parameter τ1 is denoted as “primary mode lifetime” and may provide a measure for the lifetime of the excess minority carriers.
In another example of a method of determining a lifetime of charge carriers according to the state of the art, the time during which a difference signal representative of the difference between the measured reflectivity and the reflectivity determined before applying the laser pulse decays by 1/e is determined, wherein e is Euler's number which is well known to persons skilled in the art. If V0 is the maximum of the difference signal, the time τe during which the difference signal decays from V0 to V0/e is measured. The time τe is denoted as “1/e lifetime” and may provide a measure for the lifetime of the excess minority carriers.
The lifetime of the excess minority carriers may be influenced both by a recombination of charge carriers on recombination centers in the volume of the wafer and by a recombination of charge carriers at the surface of the wafer. The recombination of charge carriers in the volume may be characterized by a volume recombination lifetime, whereas the recombination of charge carriers at the surface may be characterized by a surface recombination lifetime.
The volume recombination lifetime is defined as the lifetime of excess charge carriers which would be obtained if recombination would occur only at recombination centers in the volume of the wafer. It is a quantity characterizing properties of the wafer material and may be substantially independent of the geometry of the wafer. The surface recombination lifetime is defined as the lifetime of charge carriers which would be obtained if recombination would occur only at the surface of the wafer. It may be influenced by the geometry of the wafer, in particular by the thickness of the wafer, wherein the surface recombination lifetime may increase if the thickness of the wafer is decreased.
If the lifetime of excess minority carriers in a wafer is determined, the measured lifetime may be approximately equal to the surface recombination lifetime, if the recombination of charge carriers occurs preferentially at the surface of the wafer. If, however, the recombination of charge carriers occurs preferentially in the volume of the wafer, the measured lifetime may be approximately equal to the volume recombination lifetime. If the rate of charge carrier recombination on the surface and the rate of charge carrier recombination in the volume are of the same order of magnitude, a measurement of the lifetime of excess minority carrier may reveal a value depending on both the surface recombination lifetime and the volume recombination lifetime.
Hence, a measurement of the volume recombination lifetime may require provisions to increase the surface recombination lifetime such that charge carrier recombination occurs preferentially in the volume of the wafer. Such provisions are conventionally denoted as “passivation of the surface”. In examples of methods of determining the volume recombination lifetime according to the state of the art, it may be desirable to obtain a surface recombination lifetime which is greater than ten times the volume recombination lifetime. In the state of the art, it has been proposed to reduce the likelihood of charge carrier recombination at the surface by increasing the thickness of the wafer, by growing a layer of silicon dioxide on the surface of the wafer, by inserting the wafer into diluted fluoric acid before performing the measurement, or by performing the measurement while the wafer is plunged into a iodine solution.
Measuring the lifetime of excess minority carriers in the layer of semiconductor material of a semiconductor on insulator structure may entail specific issues associated therewith, as will be explained in the following.
First, a laser pulse in the ultraviolet range may be used to selectively create excess minority carriers in the layer of semiconductor material.
If a laser pulse having a relatively long wavelength, for example a laser pulse in the near infrared region of the electromagnetic spectrum, is applied to the semiconductor on insulator structure, both the semiconductor layer and the substrate may be irradiated by the laser pulse, since the absorption coefficient of semiconductor materials such as silicon for near infrared light is relatively low such that the laser pulse may penetrate the layer of semiconductor material. Hence, excess minority carriers, which may influence the reflectivity of the semiconductor on insulator structure for microwave radiation, are created both in the substrate and in the layer of semiconductor material.
If a measurement of the lifetime of excess minority carriers in a semiconductor on insulator structure would be performed by means of a laser pulse comprising infrared light, both the excess minority carriers in the layer of semiconductor material and the excess minority carriers in the substrate would contribute to the reflectivity of the semiconductor on insulator structure for the microwave radiation, which would make it difficult to separate the contribution of the layer of semiconductor material from the contribution of the substrate.
Ultraviolet radiation, however, may be absorbed to a relatively large extent in the layer of semiconductor material, such that excess minority carriers may substantially be created only in the layer of semiconductor material. Hence, an influence of the substrate on the results of the measurement may be reduced.
Moreover, charge carriers in the layer of semiconductor material may recombine both at the surface of the layer of semiconductor material and at the interface between the layer of semiconductor material and the substrate. Hence, in order to measure the volume recombination lifetime in the layer of semiconductor material, it may be desirable to passivate the surface of the layer of semiconductor material to reduce the likelihood of charge carrier recombination at the surface, and to make provisions to reduce the likelihood of charge carrier recombination at the interface between the layer of semiconductor material and the layer of insulator material.
In the state of the art, it has been proposed to apply passivation techniques similar to those described above to the surface of the layer of semiconductor material, and to apply a bias voltage between the layer of semiconductor material and the substrate. For this purpose, a ground contact may be provided at the surface of the layer of semiconductor material, and the substrate may be connected to a voltage source.
A polarity of the bias voltage may be such that an accumulation of the majority carriers is created in the layer of semiconductor material at the interface between the layer of semiconductor material and the insulator layer, or such that an inversion is created at the interface, wherein a density of the majority charge carriers at the interface is reduced. If an accumulation is created, the electrical field at the interface may drive the minority charge carriers away from the interface. If an inversion is created, a density of recombination partners for minority carriers at the interface is reduced. Therefore, both an accumulation and an inversion may help to reduce a likelihood of recombination at the interface between the layer of semiconductor material and the insulator layer.
A problem of the method of measuring a lifetime of charge carriers in a semiconductor on insulator structure according to the state of the art is that the electrical field created by the bias voltage may be properly defined only in the vicinity of the ground contact. Hence, while a proper accumulation or inversion may be obtained at the interface between the layer of semiconductor material and the insulator layer in the vicinity of the ground contact, this may not be the case at a distance to the ground contact. Hence, in portions of the semiconductor on insulator structure located at a distance to the ground contact, an undesirably high likelihood of charge carrier recombination at the interface between the layer of semiconductor material and the insulator layer may be obtained.
Another problem of the method of measuring a lifetime of charge carriers in a semiconductor on insulator structure according to the state of the art is that the electrical field applied between the ground contact and the substrate may create a leakage current through the insulator layer. Since a relatively high bias voltage may be required in the method of measuring a lifetime of charge carriers according to the state of the art, a relatively high leakage current may be obtained, and new leakage paths through the insulator layer may even be created. Moreover, leakage may occur at the edges of the semiconductor on insulator structure. The leakage current may adversely affect the measurement of the charge carrier lifetime, since it may provide additional carriers replacing those that have recombined (leading to wrongly overestimated lifetime values) or may remove carriers that have not recombined yet (leading to wrongly underestimated lifetime values).
It is an object of the present invention to provide an apparatus and a method for measuring a lifetime of charge carriers wherein the above-mentioned problems may be avoided or at least reduced.